Tractor frames to which fifth wheels are secured generally include a pair of longitudinally extending and transversely spaced channel members of C-shaped cross section. The fifth wheels are mounted on the channels by brackets. The brackets are supported by the upper horizontally projecting flange of the channels and are required to transmit (a) lateral inertial forces and moments from curving and rocking of the trailer, (b) longitudinal forces and moments from acceleration, deceleration and downward forces on the ramps eccentric to the rear of the bracket pivot center while preparing to couple. These conditions can produce locally high lifting forces between the bracket and the tractor frame. The brackets are constructed to permit limited longitudinal tilting of the fifth wheel relative to the frame.
In order to minimize twisting of the frame and channels, it is desirable that loads assumed by the frame through the fifth wheel be transmitted near to the shear center of the total frame structure and near to the vertical web of each channel. Heretofore various structural arrangements have been used in an endeavor to achieve such a result.